The present invention generally relates to an image-forming system and more particularly, to a developing apparatus for forming a toner image on a photoreceptor or recording medium through employment of a magnetic brush, and a developing process employing said developing apparatus.
Conventionally, as a developing process of a magnetic brush type as referred to above, there has been well known a practice as shown in FIGS. 1 and 2, in which a developing sleeve 2 incorporated therein with a magnet roller 3 is provided in a position confronting the photosensitive surface of a photoreceptor drum 1 driven for rotation in the direction indicated by an arrow (a), and while the developing material supplied to a developing material supply portion F by a bucket roller 4 is being held on the outer peripheral surface of the developing sleeve 2 by the magnetic force of the magnet roller 3, said developing material is transported along the outer peripheral surface of the developing sleeve 2 in the direction indicated by an arrow (c) based on the rotation of the magnet roller 3 in the direction of an arrow (b) and the rotation of the developing sleeve 2 in the direction of the arrow (c), thereby to develop an electrostatic latent image formed on the photosensitive surface of the photoreceptor drum 1 at a developing region V where the surfaces of the developing sleeve 2 and the photoreceptor drum 1 are located close to each other. The arrangement of FIGS. 1 and 2 further includes a magnetic brush bristle height restricting plate 5 disposed above and close to the surface of the developing sleeve 2, a scraper 6 disposed below the developing sleeve 2 in contact with the surface thereof, and a power supply 7 for applying a developing bias to the sleeve 2, with said developing sleeve 2 also functioning as a developing electrode.
With respect to the type in which the magnet roller 3 is driven for rotation in the direction from the developing region V toward the developing material supply portion F as described above, the type in which the developing sleeve 2 is driven for rotation in the opposite direction, i.e. in the developing material transport direction on the whole, is referred to as an FF system, while the type in which the developing sleeve 2 is rotated in the same direction as that of the magnet roller 3 is called an FR system hereinafter. One example of the developing method by the FR system is disclosed, for example, in U.S. Pat. No. 4,267,248.
Incidentally, in the conventional developing processes (not limited to the FF system, but including the FR system or a system in which only either one of the developing sleeve or the magnet roller is driven for rotation), after passing through a position X1 (interval or distance d1) where the surfaces of the developing sleeve 2 and the photoreceptor drum 1 are the closest to each other, the developing material completes the contact with respect to the surface of the photoreceptor drum 1 at a position X1' (interval or distance d1') where the developing sleeve 2 is leaving the surface of the photoreceptor drum 1. In the known arrangement as described above, however, there are such problems as fogging of toner respect to developed images, leakage of toner out of the developing apparatus, and in the case of a developing material employing carrier, such inconveniences as of toner images, faulty reproduction of fine lines, or adhesion of carrier with a small magnetic restriction onto the photoreceptor drum, etc., resulting from scraping off of the toner images by the carrier.
It has been confirmed that the problems as described above are attributable to the fact that the distance d1' is larger than the distance d1, and that in the developing material employing carrier, fogging and leakage of toner tend to occur in the presence of toner with a small electrical charge amount within the developing material. Furthermore, in the developing material adopting the carrier, it is unavoidable that the toner with a small charge amount is present in the developing material at a constant ratio, even if small in the quantity.
The toner with a small electrical charge amount as referred to above is liable to leave the carrier for floating, and thus, when the developing material is about to leave the surface of the photoreceptor drum 1, the toner with the small amount of electrical charge is to float in the space at the position X1' described earlier.
As is clear from the conditions d1'&gt;d1 stated previously, the electric field between the surfaces of the developing sleeve 2 and the photoreceptor drum 1 at the downstream side in the transport direction beyond the position X1' and the position in the vicinity thereof is not very strong, being weaker than that in the vicinity of the closest position X1. Accordingly, the floating toner can not fully receive the electric force from the electrostatic latent image formed on the surface of the photoreceptor drum 1, and consequently, adheres to the background portion of the image to form the undesirable fogging, or leaks out downwardly without being attracted onto the developing sleeve 2 or photoreceptor drum 1.
Meanwhile, the fact that the electric field by the electrostatic latent image at the position X1' is weaker than that at the closest position X1, means that the attraction toward the photoreceptor drum side with respect to the toner adhering to the surface of the photoreceptor drum 1 at the closest position X1 becomes weaker at the position X1' than in said closest position X1. Accordingly, there occurs such a phenomenon that the toner adhering to the image portion at the closest position X1 is scraped off by the brush bristles of the developing material (carrier) at the position X1', and such a phenomenon leads to blurring of toner images or faulty reproduction of fine lines in the toner images.
In addition, if the carrier particles have small diameters and are of binder type, they are small in the magnetic force and capable of possessing a considerably high charge amount opposite in polarity to the toner, and thus, such carrier particles tend to adhere to the image background portion. Such phenomenon is liable to take place as the interval or distance d1' becomes larger due to reduction of the magnetic attracting force by the magnet roller 3.